This work was supported by the Deutsche Forschungsgemeinschaft (grant nos.: YI 96/3-1 and MU 1791) and the European Union (ITN-SBMP). We thank C. Bippes and S. Mari for assistance and critical reading.

Keywords:

barrel structures;

mechanical unfolding;

membrane proteins;

molecular interactions;

protein unfolding

Roll out the barrel: By using single-molecule force spectroscopy, a β-barrel-forming outer-membrane protein is unfolded for the first time. OmpG from E. coli shows a surprising unfolding behavior: Single β strands do not unfold individually but as β hairpins. These β hairpins unfold one after another until the entire β-barrel membrane protein is unfolded (see structural representation).